Self-return swivel of opposed plate type



1965 A. v. MILLER ETAL 3,199,826

SELF-RETURN SWIVEL 0F OPPOSED PLATE TYPE Filed Jan. 15, 1964 2Sheets-Sheet l INVENTORS. AUSTIN v. MILLER- HOWARD s. HUNT &

BY EVERETT THOMPSON ATTORNEY 10, 1965 A. v. MILLER ETAL 3, 99,8 6

SELF-RETURN SWIVEL OF OPPOSED PLATE TYPE Filed Jan. 13, 1964 2Sheets-Sheet 2 RvA Q INVENTORS. AUSTIN V. MILLER, HOWARD S. HUNT & BEVERETT L. THOMPSON Onmmt ATTORNEY United States Patent f SELF-RETURNSWIVEL 0F @PPWSED PLATE TYPE Austin V. Miiler 23d Howard 5. Hunt,Louisville, Ky, and

Everett 1.. Thompson, New Albany, 11:11., assignors to MiddletownManufacturing (30., inc, Middletown, Ky,

a corporation of Kentuciny Filed Jan. 13, 1964-, Ser. No. 337,363 10Claims. (Cl. 248-417) This invention relates to self-return swivels.

Counter stools of the type having a back, i.e., a member providingsupport for the back of an occupant, are customarily provided withswivels. For various reasons, it is desirable to provide swiveledcounter stools with self-return means urging the stool toward, andyieldably holding it in a neutral position. The satisfactoryaccomplishment of this desideratum has been found difficult in swivelsof the opposed plate type, wherein the upper plate is mounted on thelower plate for horizontal rotary movement about a vertical axis and isvertically spaced from the lower plate slightly but sufiiciently toprovide the clearance required for such horizontal rotary movement.

The principal object of the present invention is, therefore, to providea highly satisfactory self-return swivel of the opposed plate type.

Other objects are: to provide a self-return swivel of the opposed platetype which is so simply constructed as to render it relativelyinexpensive to manufacture and assemble; to provide one which, whenassembled, may be easily and quickly installed; to provide one which,when installed, provides a smooth quiet operating action in a sturdyconstruction requiring relatively little maintenance; and to provide onewhich, when maintenance is required, may be readily removed and eitherrepaired or replaced at low cost.

A specific object of this invention is to provide a preferred form ofself-return swivel of the opposed plate type which utilizes a springlong enough to insure balanced spring pressure in opposite directions inthe neutral position and smooth spring pressure in either direction overan operating an le of substantial value, say 90 more or less in eachdirection.

The foregoing objectives of my invention are simply and fullyaccomplished in the preferred embodiment of my invention by modifying aswivel of the opposed plate type in the following manner, viz.: (1)providing the inner face of the lower plate with an upwardly-opengroove, which extends in a complete circle about said axis; (2)providing the inner face of the upper plate with a correspondingdownwardly-open circular groove, which cooperates with the lower grooveto form a longitudinal circular passageway; (3) providing the lowerplate with a spring-stopping abutment which extends across the lowergroove within the confines of the lower plate; (4) providing the upperplate with a spring-compressing abutment, which extends across the uppergroove within the con fines of the upper plate and which, in the neutralposition, is vertically aligned with the lower abutment but verticallyspaced therefrom sufiiciently to permit the movement of the upperabutment in either direction; and (5) providing the circular passagewayformed by these grooves with a helical spring which lengthwise extendsthe full length of the circular passageway from one side of neutral tothe other and which diameter-wise fills both halves of the longitudinalpassageway sufiiciently to place a substantial portion (a) of its lowerhalf at each end in axial face-to-face relationship with the adjacentaxial face of the lower spring-stopping abutment and (b) of its upperhalf at each end in axial face-to-face relationship with the adjacentaxial face of the upper sprin -compressing abutment.

Patented Aug. 10, 1965 In the preferred embodiment, the length of thecircular passageway approximates the circumference of a complete circlewhile the length of the spring is slightly greater so that it must becompressed somewhat in order to be installed within that passageway.With this arrangement, the initial compression of the spring provides abalanced spring pressure in opposite directions in the neutral positionof the swivel while its substantial length insures the production of asmooth operating action in each direction over a substantial operatingangle of more or less. In other words, a spring 360 long when slightlycompressed .permits the securement of optimum (or at least highlysatisfactory) performance over a seat moving angle of 200 from oneextreme limit to the other.

Springs shorter than 360 can be employed with a somewhat proportionalsacrifice in performance. This reduction must be kept within practicallimits and, when this is done, the sacrifice in performance may beminimized if the ratio of spring length in degrees to the angle of seatmovement remains at a value approximating 360:200 or 1.81.0.Neverthless, while springs shorter than 270 may be used in some cases,they are not recommended for general use.

In the preferred embodiment the lower spring-stopping abutments and theupper springstopping abutments lie on opposite sides of a horizontalplane passing centrally through the clearance space between the plates.However, these stop abutments may project through that plane so long asthe lower spring-stopping abutments are out of the horizontal rotarypath of movement of the upper spring compressing abutments. In anyevent, it will be appreciated that the use of our invention involvesonly the simplest of inexpensive changes in the manufacture of theswivel and that these changes do not complicate the manufacture,assembly, operation or maintenance of the construction or otherwiseappreciably increase the cost thereof.

The invention is illustrated in the drawings wherein:

FIG. 1 shows a conventional counter stool in its normal relationship toa conventional food-service counter;

PEG. 2 is a top plan view of a self-return swivel made in accordancewith our invention, the top plate in this view being rotated 45counter-clockwise out of its neutral position and being partly broken toshow the self-return spring and its relationship to the spring-stoppingabutment of the lower plate and the spring-compressing abutment of theupper plate;

FIG. 3 is a vertical central section taken along line 33 of FIG. 2, thisview showing how the lower springstopping abutment blocks the lower halfof one end of the spring;

FIG. 4 is an exploded view of the swivel as seen in FIG. 3 except thatthe upper plate is now shown in its neutral position;

H6. 5 is a fragmentary vertical section taken centrally through thecenter portion of the swivel to show the relationship of the cooperatingupper and lower abutments when the upper plate has been rotated out ofits neutral position over an angle of desired maximum value, say

FIG. 6 is a fragmentary section corresponding to one taken through theswivel along the right end portion of line 33 of FIG. 2 when the upperplate is in tis neutral position, this view showing the neutral positionrelationship of the upper spring-compressing abutment and the lowerspring-stopping abutment;

FIG. 7 is a somewhat schematic view illustrating the relationship of thespring-stop and spring-compressing abutments to the spring;

FIG. 8 is a schematic view illustrating a modification in a mannersimilar to that of FIG. 7; and

FiG. 9 is a fragmentary view of a modified arrangement of thespring-stopping and spring-compressing abutments adjacent one or bothends of the spring.

A counter stool is shown in FIG. 1 as it appears when conventionallypositioned before a conventional service counter 1. This counter stoolconventionally comprises a pedestal base 2; a seat 3 having a back 4;and a selfreturn swivel 5 mounting the seat 3 upon the upper end of thepedestal base 2 for limited rotational or angular movement about thevertical axis of the pedestal.

The self-return swivel 5 holds the seat 3 in its neutral positionwherein it faces the counter 1. An occupant, desiring to occupy theseat, will normailly rotate the seat about 90 out of neutral, occupy itand then turn to face the counter 1, during which the occupant normallyrotates the seat back to its neutral position. Upon leaving the seat,the occupant will once again rotate it approximately 90 but this time,when the occupant releases the seat, it will automatically return to theneutral position shown.

FIGS. 2-4 show a swivel of the opposed plate type which may be made inaccordance with any of the conentional swivel designs such as thatdisclosed in U. S. Patent No. 3,025,116. The particular swivel plateillustrated is conventional to the extent that it includes: a pair ofsheet metal plates; outer annular ball bearing means; andcentrally-disposed plate-securing means.

The plates consist of a lower stationary plate 7 and an upper rotaryplate 8 assembled'in spaced face-to-face relationship. The saidstationary lower plate 7 is adapted for firm attachment to saidstationary pedestal base 2. The rotary upper plate 8 is adapted for firmattachment to said rotary seat 3. Both plates have (a) aligned centerholes with a common vertically extending axis, (b) correspondingrelatively-yieldable concentric center portions C located immediatelyadjacent said center holes, (c) corresponding concentric outer portions0 located along the peripheries of the plates and spaced outwardly asubstantial distancefrom said concentric center portions, and (d)corresponding concentric intermediate portions I located between saidconcentric center and outer portions.

The outer annular ball bearing means 9 is sandwiched between saidconcentric outer portions 0 of said spaced plates to facilitate thespacing of said plates and the antifrictional rotation of therotaryplate relative to the stationary plate about said common verticalaxis.

The centrally disposed plate-securing means adjustably secures theplates in assembled relationship for relative rotation about saidvertical axis. It includes (1) a stub shaft 16 extending verticallythrough the center holes of the plate and (2) a pair of clamping membersmounted on opposite ends of the shaft for relative adjusting movementtoward and away from each other to clamp the yieldable center portionsof the plate in assembled relationship with a clamping pressure whichmay be adjusted in one direction and the other to increase and decreasethe force required to produce relative rotation between the plates. Thelower end of said adjustable securing means is in the form of anenlarged head 11, which is located at and formed integrally with thelower end of the stub shaft 10. The upper end of said adjustablesecuring means includes (1) an adjustable clamping mit 12 threaded tothe upper end of the stub shaft 16 and (2) a center ball bearingassembly which is interposed beween said adjustable clamping nut 12 andthe outer face of said concentric center portion of the adjacent upperswivel plate 8 so as to transmit the claming pressure from said clampingnut 12 to said adjacent plate 8 through rolling bearing engagement andthereby facilitate relative rotation therebetween. The center ballbearing assembly includes a circular series of ball bearings 13, anupper ball bearing race formed on the underside of the adjustableclamping nut 12 and a lower ball bearing race provided by a washer-likemember 14 encircling 4% stub shaft 1%) adjacent to but se *aratelyfabricated from said upper plate 8.

Stated somewhat spechically, the preferred embodiment of our improvedarrangement for urging the rotary upper piate toward and yieldabiyholding it in a neutral position comprises: groove-providing means;spring-return means; and a stop means. The groove-providing meansprovides the inner faces of said lower and upper plates with opposed(lower and upper) grooves 1'7 and 18 cooperativey forming between saidplates a. longitudinal passageway 19 which extends circularly about saidaxis.

The spring-return means urges said rotary plate toward, and yieldablyholds it in, its neutral position. The springreturn means includes: aspring; spring expansion-stopping means; and spring-compressing means.The spring means is in the form of an elongate helicar spring 2%)positioned between the inner faces of said plates '1' and 3 arranged toextend longitudinally in and along said circular passageway 19. it hasits upp rmost and lowermost portions in the upper and lower plate partsof said passageway 19. The spring-expansion-stopping means 21 is mountedon the lower stationary plate and arranged to present a pair of oppositeside faces providing stationary lower spring expansion-stoppingabutments within the confines of the lower groove 17. One springexpansion-stopping abutment axially faces the lower half of one end ofspring 2%) and the other axially faces the lower half of the oppositeend of spring 29. The spring-compressing means is mounted on the upperrotary plate for movement therewith and arranged to present a pair ofopposite side faces providing upper spring-compressing abutments withinthe confines of the upper groove 18. One spring-compressing abutmentaxially faces the upper half of said one end of spring 20 and isoperative, when said rotary plate is moved in one direction out of itsneutral position, to compress said spring against the stationaryspring-stopping abutment 21 at the other end of the spring. The otherspring-compressing abutment 22 axially faces the upper half of the otherend of spring 29 and is operative, when said rotary plate is moved inthe opposite direction out of its neutral position, to compress saidspring 29 against the stationary spring-stopping abutment 21 at said oneend of the spring.

The stop means functions to limit the angular movement of the upperrotary plate 3 in each direction out of its neutral position. Itincludes a spaced pair of stationary lower abutments 25 and 27 on thelower plate 7 and an upper abutment 26 interposed between lowerabutments 25 and find carried by the upper plate 8 in position to engageone lower plate abutment 25 when the upper plate is rotated in onedirection to a predetermined extent out of its neutral position and theother lower plate abutment 27 when the upper plate 8 is rotated in theopposite direction a predetermined extent out of its neutral position.

The passageway 19 preferably is located in the outer portion 0 of theswivel plate in order to maximize its length and thereby permit the useof a long spring 29 as to maximize the operating angle and promotesmoothness in the operating action of the swivel. This passagewaypreferably is located adjacent the circular passageway for the ballbearings 9 so as to avoid clamping the spring 2% between the yieldableplates 7 and 8.

The spring 2% should be longer than the length of the passageway 19 inwhich it is fitted. The excess length of the spring should be such as toinsure a firm balanced pressure which tends to hold the rotary plateyieldably in its neutral position. The magnitude of this excess lengthmay vary, depending upon the other specifications of the spring.

The stationary spring-stopping abutment 21 preferably is located at oneside or the other of the stationary plate as distinguish-ed from thefront side thereof which would correspond to the front side of the seat3.

In the construction shown, the movement limiting abutments 2527 arelocated in the intermediate section of the swivel but they may belocated wherever they can be accommodated. These abutments may beconventionally formed simply by pressing metal tongues out of eachplate. They should, of course, be so located as to prevent the springfrom being compressed to an excessive degree. However, the preferredarrangement of FIGS. 27, readily accommodates a rotary movement somewhatgreater than 100 in one direction but, since 100 is sufficient to meetsubstantially all of the reasonable rotary movement requirements ofself-return swivels, the stops -26 and 26-27 are positionedcorrespondingly. Thus, in FIG. 2, wherein the rotary plate has beenrotated counter clockwise, it will be appreciated that it must berotated in the counter clockwise direction more than an additional 45 inorder to engage the counter clockwise stop on the lower plate.

In the preferred form, good results have been obtained with a square 7/4" x 7%" x /s" self-return swivel plate of the character shown; a platethickness approximating /s"; a cross-sectional inside diameter of in thepassageways for the ball bearings 9 and for the spring 29; a clearanceof .116 between the inner faces of the plates along each side of thepassageways for bearings 9 and spring 21 a clearance of 4 between thestationary spring-stopping and rotary spring-compressing abutments inthe neutral position; an oil tempered spring having a diameter of .335and a relaxed length of 17.75" to 18.0, said spring approximating 7 /3turns per inch of .054" steel wire; and a spring-accommodating lengthapproximating 15.7" in passageway 19.

In the FIGS. 27 construction, the lower plate engages the lowerhorizontal half of the spring 28 while the up or late engages the upperhorizontal half thereof. This construction is schematically illustratedin FIG. 7, but, for the sake of clarity, PlG. 7 shows the lower plateengaging the inner vertical half. FIG. 7 also portrays thespring-stopping and spring-compressing means 21-22 on each plate as ifeach of them was somewhat divided to provide two separate abutments, onefor clockwise and the other for counter clockwise action. This is doneto emphasize the fact that the number and location of the stops on oneplate or the other may be varied and this is particularly true where ashorter spring 20 is used.

In accordance with another feature of our invention, access to circularpassageway 19 for spring assembly and removal purposes is created byproviding one or the other of the swivel plates 7 and 8 with an accessopening on the spring engaging side of its spring engaging abutment.Thus, in FIG. 2, the top plate 8 is provided with an opening 29providing access to the passageway 19 on and adjacent to thecounter-clockwise side of its stop 22. A similar opening may be providedon and adjacent to the clocl'wise side of that stop but, since it is notnecessary, it is not shown. With this arrangement, the swivel may becompletely assembled in every respect except for the assembly of thespring 20. To complete the assembly in all respects, the spring 20 canbe inserted into passageway 19 through access opening 29. Whenevernecessary, it may be removed through that opening without dismantlingthe swivel.

FIG. 8 is a schematic representation similar to 1G. 7 of a modificationof the FIG. 2-7 structure. In this modification a spring having arelaxed length approximating 13 /2 is compressed into a circularpassageway (such as 19 of FIGURE 6) which has a spring accommodatinglength approximating 11.8" and extending over an angle of 270. if theratio of spring length in degrees (270) to the angle of seat movementremains at a value of 1.811, then the angle of seat movement willapproximate 150. This angle may, however, be increased to at least 180and possibly 200. In FIG. 8, two springstopping and twospring-compressing abutments are provided. For example, in the neutralposition of the swivel, one spring-stopping abutment 219 is verticallyaligned at one end of the spring 2% with one spring-compressing abutment22% whi e the other spring-stopping abutment 211 is vertically alignedat the opposite end of spring 200 with the other spring-compressingabutment 221. If some idling movement of the FIG. 8 rotary plate can betolerated, when the swivel is unrestrained, the spacing between theupper or rotary plate abutments 220 and 221 may be reduced. If reducedto 60, the rotary swivel can swing idly through an angle of 30 withoutundergoing any spring action. With only one upper abutment, the rotaryplate can swing idly through an angle approximating 90.

in the FIG. 2-7 construction and in the FIG. 8 modification thereof, thespring-stopping abutments remain within the confines of the lower platewhile the springcompressing abutments remain within the confines of theupper plate. In other words, the lower stop 21 (or 21%) is spaced belowand the upper stop 22 (or 220) is spaced above a horizontal planecentrally disposed between plates. A modfication of this arrangement isillustrated in PEG. 9 wherein one plate, preferably the lower plate '7,is provided with a spring-stopping abutment in the form of a screw 33.0which projects upwardly through passageway it) in position to abut oneportion of one end face of said spring while the upper plate issimilarly provided with one abutment in the form or" screw 32%projecting downwardly through passageway 19 in position to abut adifierent portion of said one end face of said spring. The bottom platepreferably has one set of two screws 3 10 in order to provide balancedspring engaging action. in the 360 arrangement of FIG. 9, two bottomscrews 31:; cooperate with one top screw 320 (or vice versa) to provideone set of spring engaging abutments accommodating movement in bothdirections. Two such sets are necessary for the 270 arrangement of FIG.8.

While the foregoing type of self-return swivel is admirably suited foruse in counter stools, it is equally Well suited for use in dinettechairs, lounge chairs and various other products. its simplicity rendersit relatively inexpensive to manufacture, assemble, install andmaintain. It provides a sturdy structure having a smooth quiet anddependable operating action which should require rela tively littlemaintenance and which, when maintenance is required, may be readilyremoved and either repaired or replaced at low cost.

The term spring-actuating abutment is intended to be identical inmeaning to the term spring-compressing abutment; hence may be usedinterchangeably therewith.

Having described our invention, we claim:

1. In a self-return swivel of the opposed plate type wherein the upperplate is vertically spaced from the lower plate and mounted thereon forhorizontal rotary movement about a vertical axis, an improvedarrangement for urging the rotary plate toward and yieldably holding itin a neutral position, comprising:

(A) means providing the inner faces of said lower and upper plates withopposed grooves cooperatively forming, between said plates, alongitudinal passageway which extends circularly about said axis; and

(B) means urging said rotary plate toward, and yieldably holding it inits neutral position, said means including (1) an elongated helicalspring extending longitudinally in and along said circular passageway,(2) spring-stopping means (a) providing a pair of stationary abutmentsmounted on the lower stationary plate, one to abut one end of saidspring and the other to abut the other end thereof, and (3)spring-compressing means (a) providing one actuating abutment, which isstationarily mounted on the upper rotary plate to abut one end of saidspring and which is operative, when the upper plate is 7 moved one wayout of said neutral position, to compress said spring in one directionagainst said other stationary abutment,

(b) providing another actuating abutment, which is stationarily mountedon the upper rotary plate to'abut the other end of said spring and whichis operative when the upper plate is moved the other way out of saidneutral position, to compress said spring in the other direction againstsaid one stationary abutment.

2. The improved arrangement of claim 1 wherein:

(A) each actuating abutment is carried by the upper rotary plate formovement along circular paths which clear the lower stationaryabutments.

3. The improved arrangement of claim 2 wherein:

(A) in the neutral position or" said swivel, said spring is partiallycompressed between said stationary abutments.

4. The improved arrangement of claim 3 wherein: (A) one of said swivelplates has an access opening to facilitate the insertion and removal ofsaid spring,

(1) said access opening being located adjacent one end of said springwhen the swivel is in its neutral position.

5. A self-return swivel of the opposed plate type, comprising:

(A) a lower horizontal stationary plate;

(B) an upper horizontal rotary plate;

(C) means rotationally mounting said upper plate on, and in verticallyspaced relationship relative to said lower plate for limited horizontalrotary movement about a vertical axis of rotation in either directionout of a neutral position;

(D) means providing the inner faces of said lower and upper plates withopposed grooves cooperatively forming between them a longitudinalpassageway which extends circularly about said axis; and

(E) means urging said rotary plate toward, and yieldably holding it in,its neutral position, said means including (1) an elongate helicalspring extending longitudinally in and along said circular passageway,(2) spring-stopping means i (a) providing a pair of stationary abutmentsmounted on the lower stationary plate,

(1) one abutting the lower half of one end of said spring and (2) theother abutting the lower half of the other end of said spring, and

(3) spring-compressing means (a) providing a pair of actuating abutmentsmounted on' the upper rotary plate for movement therewith,

(1) one abutting the upper half of said one end of said spring and beingoperative, when said rotary plate is moved in one direction out of itsneutral position, to compress said spring against said other stationaryspring-stop abutment, and

(2) the other abutting the upper half of said other end of said springand being operative, when said rotary plate is moved in the otherdirection out of its neutral position, to compress said spring againstsaid one stationary spring-stop abutment.

6. The swivel of claim 5 wherein:

(A) in the neutral position of said swivel, said spring is partiallycompressed between said stationary abutments; and

(B) one of said swivel plates has an access opening for the insertionand removal of said spring,

(1) said access opening being located adjacent one end of said springwhen the swivel is in its neutral position.

7. The swivel of claim 6 wherein:

(A) said spring-stopping means on the lower plate is located at theneutral position of said swivel; and (B) when said upper plate is in itsneutral position, said spring extends continuously approximately 360from one side of said spring-stopping means to the other side thereof.

3. The swivel of claim 7 wherein:

(A) when said upper plate is in its neutral position, saidspring-compressing means is vertically aligned with and verticallyspaced from said spring-stopping means.

9. A self-return swivel of the opposed plate type having an elongatehelical spring for urging the rotary plate toward and yieldably holdingit in a neutral position, comprising:

(A) a lower horizontal stationary plate;

(B) an upper horizontal rotary plate;

(C) means rotationally mounting said upper plate on, and in verticallyspaced relationship relative to said lower plate for limited horizontalrotary movement about a vertical axis of rotation in either directionout of a neutral position;

(D) means providing the inner faces of said lower and upper plates withopposed grooves cooperatively forming between them a longitudinalpassageway which extends circularly about said axis;

(E) spring-stopping means (1) providing a pair of stationary abutmentsmounted on the lower stationary plate,

(a) one to abut the lower half of one end of a spring in saidpassageway, and

(b) the other to abut the lower half of the other end of said spring;and

(F) spring-compressing means (1) providing a pair of actuating abutmentsmounted on the upper rotary plate for movement therewith,

(a) one to abut the upper half of said one end of a spring in saidpassageway and being operative, when said rotary plate is moved in onedirection out of its neutral position, to compress said spring againstsaid other stationary spring-stop abutment, and

(b) the other to abut the upper half of the other end of a spring insaid passageway and being operative, when said rotary plate is moved inthe other direction out of its neutral position, to compress said springagainst said one stationary spring-stop abutment.

10. The swivel of claim 9 wherein:

(A) one of said swivel plates has an access opening to facilitate theinsertion and removal of said spring,

(1) said access opening being located adjacent one end of saidpassageway when the swivel is in its neutral position.

References Cited by the Examiner UNITED STATES PATENTS 1,782,270 11/30Mendenhall 248417 2,217,943 10/40 Carlson 248-418 2,329,697 9/43 Davies248417 2,413,455 12/46 Kiefer 248-417 2,691,407 10/54 Kupski 2484172,779,642 1/57 Matthews 248349 CLAUDE A. LE ROY, Primary Examiner.

1. IN A SELF-RETURN SWIVEL OF THE OPPOSED PLATE TYPE WHEREIN THE UPPERPLATE IS VERTICALLY SPACED FROM THE LOWER PLATE AND MOUNTED THEREON FORHORIZONTAL ROTARY MOVEMEMT ABOUT A VERTICAL AXIS, AN IMPROVEDARRANGEMENT FOR URGING THE ROTARY PLATE TOWARD AND YIELDABLY HOLDING ITIN A NEUTRAL POSITION, COMPRISING: (A) MEANS PROVIDING THE INNER FACESOF SAID LOWER AND UPPER PLATES WITH OPPOSED GROOVES COOPERATIVELYFORMING, BETWEEN SAID PLATES, A LONGITUDINAL PASSAGEWAY WHICH EXTENDSCIRCULARLY ABOUT SAID AXIS; AND (B) MEANS URGING SAID ROTARY PLATETOWARD, AND YIELDABLY HOLDING IT IN ITS NEUTRAL POSITION, SAID MEANSINCLUDING (1) AN ELONGATED HELICAL SPRING EXTENDING LONGITUDINALLY INAND ALONG SAID CIRCULAR PASSAGEWAY, (2) SPRING-STOPPING MEANS (A)PROVIDING A PAIR OF STATIONARY ABUTMENTSD MOUNTED ON THE LOWERSTATIONARY PLATE, ONE TO ABUT ONE END OF SAID SPRING AND THE OTHER TOABUT THE OTHER END THEREOF, AND